The present invention relates to a high frequency switching circuit, and more particularly to a microwave electronic switching circuit using field effect transistors.
One of the two-input one-output high frequency switching circuits using field effect transistors will be described with reference to FIG. 1. The high frequency switching circuit has first, second and third signal terminals "A", "B" and "C". A first field effect transistor Q7 is provided in series between the first and third signal terminals "A" and "C". A second field effect transistor Q9 is provided in series between the first signal terminal "A" and a ground line. A third field effect transistor Q8 is provided in series between the second and third signal terminals "B" and "C". A fourth field effect transistor Q10 is provided in series between the second signal terminal "B" and the ground line. The high frequency switching circuit has first and second control bias terminals "VA" and "VB". A first resistor "RG1" is provided between the first control bias terminal "VA" and a gate terminal of the first field effect transistor Q7. A second resistor "RG2" is provided between the second control bias terminal "VB" and a gate terminal of the third field effect transistor Q9. A third resistor "RG3" is provided between the second control bias terminal "V3" and a gate terminal of the second field effect transistor Q8. A fourth resistor "RG4" is provided between the first control bias terminal "VA" and a gate terminal of the fourth field effect transistor Q10.
The first and second signal terminals "A" and "B" are the signal input terminals to which input signals are inputted. The third signal terminal "C" is the signal output terminal from which an output signal is outputted. If the first and third field effect transistor Q7 and Q9 are in the OFF state, then the input signals having been inputted through the first and second signal terminals "A" and "B" are cut off whereby the above switching circuit enters into OFF state. When the above switching circuit is in the OFF state, then the second and fourth field effect transistors Q8 and Q10 turn ON for attenuation of the leakage of signals If, however, the first and third field effect transistor Q7 and Q9 are in the ON state, then the input signals having been inputted through the first and second signal terminals "A" and "B" are transmitted to the third signal terminal "C" whereby the above switching circuit enters into ON state. When the above switching circuit is in the ON state, then the second and fourth field effect transistors Q8 and Q10 turn OFF. Gate control signals are applied to the first and second control bias terminals "VA" and "VB" so that the gate control signals are transmitted through the fast, second, third and fourth resistors RG1, RG2, RG3 and RG4 to the gates of the first. second, third and fourth field effect transistors Q7, Q8, Q9 and Q10.
If the first control bias terminal VA is applied with a gate control signal having a lower voltage level than a predetermined threshold voltage level and the second control bias terminal VB is applied with another gate control signal having a higher voltage level than the predetermined threshold voltage level, then the first and fourth field effect transistors Q7 and Q10 turn OFF whilst the second and third field effect transistors Q8 and Q9 turn ON. Namely, the first input signal having been inputted into the first signal terminal "A" is cut off whilst the second input signal having been inputted into the second signal terminal "B" is thus transmitted to the third signal terminal "C". In this case, the second field effect transistor Q8 is in the ON state so that the first input signal is transmitted to the ground line whereby the input signal having a high frequency is attenuated. As a result, a signal isolation or attenuation of not less than 30 dB can be ensured between the first and third signal terminals "A" and "C". This means that the first signal terminal "A" is short-circuited.
If the first control bias terminal VA is applied with a gate control signal having a higher voltage level than the predetermined threshold voltage level and the second control bias terminal VB is applied with another gate control signal having a lower voltage level than the predetermined threshold voltage level, then the first and fourth field effect transistors Q7 and Q10 turn ON whilst the second and third field effect transistors Q8 and Q9 turn OFF. Namely, the first input signal having been inputted into the first signal terminal "A" is thus transmitted to the third signal terminal "C" whilst the second input signal having been inputted into the second signal terminal "B" is cut off. In this case, the fourth field effect transistor Q10 is in the ON state so that the second input signal is transmitted to the ground line whereby the second input signal having a high frequency is attenuated. As a result, a signal isolation or attenuation of not less than 30 dB can be ensured between the second and third signal terminals "B" and "C". This means that the second signal terminal "B" is short-circuited.
If the first signal terminal "A" is short-circuited, then in order to have an input impedance accord to a characteristic impedance Z0, typically 50 .OMEGA., 75 .OMEGA., in the signal source side based upon the first signal terminal "A", the ON-resistance of the second field effect transistor Q8 is required to be set Z0. Actually, however, the ON-resistance of the second field effect transistor Q8 is larger by ten times or more than Z0, for which reason if the frequency of the input signal is not less than 1.5 GHz, then the signal isolation or attenuation is considerably reduced down to 20 dB.
In order to settle the above problem with such considerable reduction in the signal isolation or attenuation, multi-stage switching circuitry was proposed but this circuitry has a serious disadvantage in increased loss of the signal intensity or amplitude.
Another conventional high frequency switching circuit will subsequently be described with reference to FIG. 2, which is also disclosed in the Japanese laid-open patent publication No. 63-142716. The high frequency switching circuit has first, second and third signal terminals "A", "B" and "C" wherein the first and second signal terminals "A" and "B" are signal input terminals into which first and second input signals are inputted, whilst the third signal terminal is an output terminal from which an output signal is outputted. The high frequency switching circuit further has first and second control bias terminals "VSA" and "VSB". A first field effect transistor "Q11" is provided in series between the first and third signal terminals "A" and "C". A gate of the first field effect transistor "Q11" is grounded. A second field effect transistor "Q12" is provided in series between the second and third signal terminals "B" and "C". A gate of the first field effect transistor "Q12" is grounded. A first resistor "R2a" is provided between the first control bias terminal "VSA" and the first field effect transistor "Q11" so that the first resistor "R2a" is connected through the first field effect transistor "Q11" to the third signal terminal "C". A second resistor "R2b" is provided between the second control bias terminal "VSB" and the second field effect transistor "Q12" so that the second resistor "R2b" is connected through the second field effect transistor "Q12"to the third signal terminal "C". However, the above switching circuitry is never the impedance matching termination switching circuit.
The above first high frequency switching circuit may be configured to be the impedance matching termination switching circuit. In this case, however, the isolation property of the circuit is remarkably deteriorated. In order to ensure the required isolation property of the circuit, the multi-stage circuit configuration is needed, resulting in an increased passing loss thereof. Further, if the ON resistances of the second and fourth field effect transistors are 50 .OMEGA., the isolation property at a higher frequency of not less than 50 GHz is deteriorated.
The above second high frequency switching circuit could never be configured to be the impedance matching termination switching circuit and also the switching circuit shows a deterioration in the isolation or attenuation at the higher frequency of not less than 50 GHz.
In recent years, micro-wave communications have been made frequently. It is likely that many devices having a plurality of local oscillators have been produced. Such local oscillator is sensitive to an external impedance, for which reason high frequency switching circuit for switching those devices is required to be impedance matching termination switching circuit having a high isolation property.